many magazine articles and transactions of scientific societies contain charts which are reproduced direct from finely ruled co-ordinate paper and show all of the lines of the co-ordinate paper in the finished illustration. Co-ordinate ruling does not appear prominently on most original charts because the ruling is usually printed in some color of ink distinct from the curve itself. When, however, a chart is reproduced in a line engraving the co-ordinate lines come out the same color as the curve or other important data, and there may be too little contrast to assist the reader.

Curves are sometimes shown plotted vertically when a horizontal arrangement could be used without any difficulty. There seems to be no

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Fig. 241. Comparison of American Automobiles for Four Years, in Cylinder Bore, Valve Arrangement, and Ignition System

There is no necessity for plotting curves in the vertical position shown here for it is only confusing to the reader. These curves cannot be read conveniently even by turning the book to read from the left because some of the type would then be upside down. See Fig. U'i